This invention pertains to electrical heater elements, and more particularly to metallic electrical heater elements with high tolerance to severe environmental influences and physical abuse, and having integral ceramic surface material for electrical insulation and chemical compatibility with many liquids and other materials in which the heater elements would be immersed or embedded in operation.
Electrical heater elements have existed for many years and the technology is well understood. The most well known material for electrical heater elements is Nichrome, a composition of 61% nickel, 24% iron, and 15% chromium. Nichrome performs adequately as an electrical heater element in numerous applications and has a low temperature coefficient of electrical resistance, but is chemically reactive, is sensitive to thermal shock, and must be electrically insulated from contact with conductive materials to prevent shorting the element or exposing people or equipment to electrical shock.
Most conventional uses of electrical heating elements use one of two basic approaches to insulating the element from contact with conductive elements in the environment. The first is to support the electrical heater element in high temperature insulators in an enclosure designed to prevent contact with the heater element by anything but the insulator and the surrounding air. The second is to encase the heater element in an electrically insulating high temperature material such as ceramic granules inside a metallic sheath. Elements of this second type are safe and relatively durable, but are expensive and burn out after a relatively short life. Moreover, they are rigid so they cannot be easily bent to conform to the item they are intended to heat.
Flexible heat tapes have been developed that can be wrapped around articles to be heated, such as water pipes to prevent them from freezing. Heat tapes have satisfied a need in many applications and are in wide use. However, heat tapes are generally low temperature devices because of the supporting fabric material to which they are connected, and the heater elements in the heater tape must be electrically insulated just like other heaters. Such electrical insulation also serves as heat insulation, so heat tapes are generally inefficient because of low heat transfer efficiency to the articles to be heated.
Thus, the art of electrical heating has long needed a simple, inexpensive heater element that is chemically non-reactive, has durable integral electrical insulation, and is insensitive to thermal shock. Ideally, such a new electric heating element would also function as a temperature indicator so that its power supply could be programmed to apply power to maintain a desired temperature or otherwise only when needed. Finally, such an ideal heater element would be flexible, and capable of being manufactured a many different sizes and forms, such as wires, flat strips and wide sheets.
Accordingly, it is an object of this invention to provide an electrical heater element that is durable and insensitive to thermal shock. Another object of this invention is to provide a method of heating using an electrical heater element in direct contact with the substrate or material to be heated.
These an other objects of the invention are attained in an electrical resistive heating system having a Nitinol heater element in the form of wire, rod strip or tube, and having connectors at each end for leads from a controller that controls the flow of current through the heater element from a source of electrical power. The Nitinol heater element is treated to have an electrically insulating surface that is also hard and chemically non-reactive, so the heater element can be put in intimate contact with the materials or substrate to be heated without shorting or electrical shock to people or equipment.